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Earnings Per Share - Additional Information (Detail) - shares
shares in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount 2.6 0.6 0.4 52.9
Restricted Stock Units (RSUs) [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount       0.5
Restricted Stock Units (RSUs) [Member] | Contingent Class A Common Stock [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount     26.7 13.0
Limited Partnership Unit [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount       52.4
Limited Partnership Unit [Member] | Contingent Class A Common Stock [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount     26.7 13.0
BGC Holdings Partnership Unit [Member] | Contingent Class A Common Stock [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount     26.7 13.0
N Units [Member] | Contingent Class A Common Stock [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Anti-dilutive securities excluded from computation of earnings per share amount     26.7 13.0